The Fischer-Tropsch reaction can be written for each carbon number n as:(2n+x)H2+nCO→CnH2(n+x)+nH2O,  (1)where x=0 and n>1 for olefins,and x=1 and n>1 for paraffins.
For an iron-based catalyst, activation of the Fischer-Tropsch reaction also activates the water gas shift reaction:H2O+CO⇄H2+CO2  (2)
The term “activation” of an iron-based FT catalyst refers to the transformation of an inert catalyst precursor such as hematite into a composition and structure that causes the reaction between hydrogen and carbon monoxide to take place at a high rate to produce predominantly hydrocarbons having more than five carbon atoms. Activation can be carried out as a one-step or two step method. In the two step method, the hematite is first reduced to Fe2O3, FeO or elemental iron using hydrogen before exposing the catalyst to either carbon monoxide or a mixture of carbon monoxide and hydrogen (synthesis gas). In the single step method the catalyst is exposed to synthesis gas without a pre-reduction step. When synthesis gas is used for activating the catalyst, the Fischer-Tropsch reaction rate increases as the carbiding reactions convert an increasing amount of oxide to carbides. This method of activating the catalyst is sometimes referred to as “induction.” A short intense activation period, which exposes a catalyst to synthesis gas for about 3 hours, is sometimes referred to as “typhoon induction.”
An iron-based catalyst that exhibits high activity for reactions (1) and (2) typically contains iron carbides including Hägg carbide (Fe5C2) and ε′-carbide (Fe2.2C). Iron carbides are formed by reacting iron or iron oxide with gases containing carbon monoxide at elevated temperatures. In the paper by Li et al., “Spectroscopic and Transient Kinetic Studies of Site Requirements in Iron-Catalyzed Fischer-Tropsch Synthesis”, J. Phys. Chem. B 2002, 106, 85-91, the authors describe the structural changes that take place with time when the catalyst precursor hematite is subjected to synthesis gas at a temperature of 523 K. From data obtained in situ on Fischer-Tropsch synthesis, Li et al. indicated that hematite, Fe2O3, is rapidly reduced to magnetite, Fe3O4, and the Fe3O4 is rapidly carburized. Since the skeletal density of the carbides is about 7.7 g/cm3 compared to 5.2 for Fe3O4, the carbide crystallites would occur in patches on the Fe3O4 core.
The carbon number distribution for the Fischer-Tropsch reaction is often characterized using the Anderson-Schultz-Flory distribution which relates successive moles of hydrocarbon as:Nn=αNn−1  (3)where Nn is the number of moles of hydrocarbon having n carbon atoms and a is a constant. Using successive substitutions, the moles at any carbon number n can be related to the number of moles of methane (n=1):Nn=αn−1N1   (4)
In reality, the carbon number distribution for an iron-based catalyst typically cannot be represented using a single value for alpha (α). At low carbon numbers, alpha values are smaller than the alpha values at high carbon numbers. However, obtaining alpha values from gas and liquid product analyses is difficult and time-consuming. Therefore, it is useful to calculate a single alpha from the data as an indicator of selectivity. For single alpha values of about 0.75 or less, very little wax, defined as hydrocarbons having 20 or more carbon atoms, is produced, whereas for single alpha values greater than about 0.85, wax is the predominant hydrocarbon product. Single alpha values can be calculated from gas chromatograph data for the inlet and outlet gas streams of the FT reactor. This technique is described in C. B. Benham, “Data analysis procedures in Fischer-Tropsch synthesis,” ACS Div. Fuel Chem. Prepr., 40(1), 1995, pp 201-202.
A useful relationship can be obtained relating single alpha to total CO conversion using the following variables:y=1/(1−ε)  (5)where ε denotes total CO conversion, andz=1/(1−α)  (6)
The following equation relates z to y and gas composition:z=√{((1+GCO2)y−RCO2−1)/(RCH4−GCH4y)}  (7)where GCO2 and GCH4 are the ratios of moles of CO2 and CH4 to the moles of CO in the inlet gases and RCO2 and RCH4 are the ratios of moles of CO2 and CH4 to the moles of CO in the outlet gases, respectively. From equation (6), the single alpha is related to z by:α=1−1/z   (8)
Therefore, from measured values of CO conversion and chromatograph data for the FT inlet and tail gases, one can determine the single alpha value as used hereinafter.
An especially effective activating procedure is described in U.S. Pat. No. 5,504,118 which issued to C. B. Benham et al., and which teaches a method of producing a catalyst having high activity and selectivity for producing predominantly hydrocarbon containing products having more than five carbon atoms. As disclosed in Example 2 of the '118 patent, the catalyst precursor is subjected to synthesis gas having a H2:CO ratio of about 1.4 at a temperature of about 280° C., at a pressure of about 150 psig, and at a space velocity of about 4 Nl/h/g Fe. The activation time was about 2 hours. The resulting catalyst exhibited high activity, high wax selectivity, and high stability. Due to the short intense typhoon activation, catalyst fines can be produced due to the rapid transformation of the crystallites from iron oxide to iron carbides. Fine catalyst particles, which can be carried over into the separated wax, can cause problems in downstream separation equipment. A new method is disclosed, which reduces generation of fine catalyst particles during activation while maintaining the activity, selectivity and stability of the catalyst described in the '118 patent. The '118 catalyst performance is used as a baseline for comparing the performance of catalysts activated using the instant method. The single alpha and CO conversion values for the baseline catalyst were about 0.85 and about 80%, respectively, under the operating conditions of the tests described hereinbelow.